swim-bladder

swim-bladder defined in 1951 year

swim-bladder - swim-bladder;swim-bladder - Bladder containing gas, present in roof of the abdominal cavity in bony fish (Actinopterygii). Functions as a hydrostatic organ, varying the specific gravity of the fish to match the depth at which it is swimming, so that the fish can remain there effortlessly. Derived from the lungs of ancestral fish, which were partly air-breathers. See also: Lung.